Angular scatter imaging system using translating apertures and method thereof

ABSTRACT

The angular imaging system, and related method utilizes translating apertures to acquire data at a number of angles of interrogation. For omnidirectional scatterers, that is scatterers which emit a uniform sound field in all directions when insonified, the translating apertures should theoretically yield identical speckle patterns at all angles of interrogation. The result is in strong contrast to previously applied convention angular scatter measurement methods which produced rapidly varying speckle patterns with interrogation angle. Thus by using the translating apertures, using the transmit aperture translator ( 62 ), and the receive aperture translator ( 72 ), it is possible to acquire data for which the only variation in received signal with angle is due to the intrinsic scattering of the target.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a national stage filing of International ApplicationNo. PCT/US00/18652, filed Jul. 7, 2000, which claims priority benefitunder 35 U.S.C. Section 119(e) from U.S. Provisional Patent ApplicationSerial No. 60/142,556 filed Jul. 7, 1999, entitled “Angular ScatterImaging Using the Translating Apertures Algorithm,” and 60/169,598 filedDec. 8, 1999, entitled “Angular Scatter Imaging Using the TranslatingApertures Algorithm” the entire disclosures of which are herebyincorporated by reference herein.

The present invention is related to Trahey et al. U.S. Pat. No.5,673,699 entitled “Method and Apparatus for Aberration Correction inthe Presence of a Distributed Abberrator” the entire disclosure of whichis hereby incorporated by reference herein.

US GOVERNMENT RIGHTS

This invention was made with United States Government support underGrant No. R01-43334, awarded by the National Institute of Health (NIH).The United States Government has certain rights in the invention.

FIELD OF THE INVENTION

The present invention is directed to a system and method for ultrasoundimaging, and particularly imaging angular scatter by coherentlyprocessing data from multiple scattering angles using translatingapertures.

BACKGROUND OF THE INVENTION

Conventional ultrasound systems transmit pulses of high frequency soundinto the body and map the magnitude of returned echoes to form B-modeimages. The brightness of these images is a function of many factorsincluding transmit and receive transducer geometry, attenuation andphase aberration in the propagation path, and most importantly, theacoustic scattering of the tissue itself. Conventional systems map theacoustic backscatter from tissue; that is the sound energy returneddirectly to the transmitter. While such images have great diagnosticvalue, they represent only a fraction of the information available fromthe scattered sound field.

One untapped source of information is angular scatter. As the incidentwave scatters from tissue structures, different fractions of its energyare scattered in different directions. Angular scatter is describedusing the geometry shown in FIG. 1. In this nomenclature backscatter isindicated by a scattering angle of 180°, while angular scatter occurs atsmaller angles. Although angular scatter information is not utilizedclinically, it has been a topic of research for over a decade. Researchin this area has consisted of both fundamental measurements and thedevelopment of practical imaging systems.

Angular scatter measurements have typically had the goal of measuringthe average angular scatter over a large area, at a single frequency asdiscussed by W. J. Davros, J. A. Zagzebski, and E. L. Madsen, inFrequency-dependent angular scattering of ultrasound by tissue-mimickingmaterials and excised tissue, Journal of the Acoustical Society ofAmerica, vol. 80, pp. 229-237, 1986, and by J. A. Campbell and R. C.Waag, in Measurements of calf liver ultrasonic differential and totalscattering cross sections, J. Acoust. Soc. Am., vol. 75, pp. 603-611,1984, the entire disclosures of which are hereby incorporated byreference herein. These systems moved piston transducers mechanicallyaround a target to interrogate different scattering angles. However, forreasons described below, these measurements exhibited large statisticalfluctuations and therefore required significant averaging to yieldreliable results. Thus, while these measurements lend insight intotissue scattering, their methods cannot be adapted for clinical imaging.

Previous angular scatter imaging systems have had the goal of imagingtissue at a single scattering angle other than 180° as discussed by M.T. Robinson and O. T. V. Ramm, in Real-Time Angular Scatter ImagingSystem for Improved Tissue Contrast in Diagnostic Ultrasound Images,IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control,vol. 41, 1994, and by J. C. Lacefield, in Angular Scatter UltrasoundImaging using Separated Arrays, Duke University, 1999 the entiredisclosures of which are hereby incorporated by reference herein. Thesesystems have used one or more phased array transducers with the transmitaperture displaced physically from the receive aperture. By applyingelectronic focusing and beam steering, these systems were able tointerrogate a 2-D region at high spatial resolution and with broadbandwidth. Angular scatter images were displayed beside accompanyingB-mode images, however direct comparison was difficult because eachimage presented a different speckle pattern. While these systems haveyielded interesting results, they do not coherently process dataacquired at different scattering angles, and thus fail to make full useof angular scatter information.

FIGS. 15(A)-(D) illustrate k-space representations of a variety ofangular scatter measurement/imaging geometries (k-space will bediscussed in greater detail below). For introduction purposes, FIG.15(A) indicates a simple backscatter geometry. The incident wave vectoris indicated by “i,” the observed wave vector by “o,” and the k-vectorby “k.” The gray oval indicates the region of k-space interrogated bythis system. This region is narrow in the axial spatial frequencydimension to indicate a narrow bandwidth. The lateral spatial frequencydimension is also narrow, indicating poor lateral spatial resolution.

FIG. 15(B) depicts the geometry used by Davros et al, as discussedabove. The dark oval indicates the region of k-space interrogated bythis system while the light oval is the backscatter system. Note thelack of overlap and thus lack of speckle coherence between thebackscatter and angular scatter interrogation.

FIG. 15(C) indicates the k-space representation of the angular scattersystem used by Campbell and Waag, as discussed above. The rotation ofDavros is eliminated by rotating the transmitter and receiver by equaland opposite increments circumferentially. However, the downshift of theaxial spatial frequencies has still eliminated any speckle coherence forthis narrowband system.

There is therefore a need in the art for an effective system and methodfor ultrasound imaging. In particular, imaging angular scatter bycoherently processing data from multiple scattering angles while stillhaving stability in the speckle pattern with angle that allows directcomparison of echoes acquired at different scattering angles using atranslating aperture.

Accordingly, FIG. 15(D) depicts the k-space representation of thetranslating apertures implemented on a broadband phased array system ofthe present invention. The broad bandwidth of this system ensures thatsome speckle coherence is maintained, even with the downshift in axialspatial frequencies.

SUMMARY OF THE INVENTION

According to the invention, a system is provided for imaging a targetusing imaging angular scattering comprising: a transducer array having aplurality of elements aligned along at least one of a plurality oftranslational axes wherein the plurality of translational axes aredirected horizontally, vertically, and/or diagonally relative to thetarget; a transmitter for generating and transmitting ultrasound pulsesat the target operably associated with the transducer array; a transmitaperture translator electrically associated with the transmitter fortransmitting pulses to fire from the elements of the transducer arraythereby defining a subject transmit aperture, wherein the subjecttransmit aperture comprises at least two preselected the elements; areceiver for receiving echoes of transmitted pulses operably associatedwith the transducer array and outputting echo signals therefrom; areceive aperture translator electrically associated with the receiverand for receiving pulses transmitted from the subject transmit apertureand received at the elements of the transducer array thereby defining asubject receive aperture, wherein the subject receive aperture comprisesat least two preselected the elements; a controller for controlling thetransmit aperture translator and the receive aperture translator whereinthe transmission and reception are iteratively performed at least twice,wherein after each of the iterations of transmission and reception thesubject transmit aperture and the subject receive aperture aretranslated along one of the plurality translation axes in apredetermined equal and opposite direction relative to one another; anda signal processor operably associated with the receiver, the signalprocessor adapted to receive the echo signals and perform angularscatter analysis on the echo signals after each of the second orsubsequent iterations so as to provide an image signal representative ofthe target.

Further, the invention provides a translating apertures method ofimaging a target comprising the steps of: a) providing a transducerarray having a plurality of elements aligned along at least one of aplurality of translational axes wherein the plurality of translationalaxes are directed horizontally, vertically, and/or diagonally relativeto the target; b) generating a subject ultrasound pulse at preselectedelements of the plurality of elements to define a subject transmitaperture; c) focusing the subject ultrasound pulse to a predeterminedpoint on the target; d) transmitting the subject ultrasound pulse fromthe subject transmit aperture to the target point; e) receiving echoesof the transmitted pulses at preselected elements of the plurality ofelements to define a subject receive aperture; f) outputting echosignals received from the receive aperture; g) repeating step “a”through step “f” at least one or more times, wherein after eachrepetition the method further comprises the additional step of:translating the subject transmit aperture and the subject receiveaperture along one of the plurality of translation axes in apredetermined equal and opposite direction relative to one another; andh) processing the echo signals to perform angular scatter analysis onthe echo signals after the first or subsequent repetitions so as toprovide an image signal representative of the target.

These and other objects, along with advantages and features of theinvention disclosed herein, will be made more apparent from thedescription, drawings and claims that follow.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of the presentinvention, as well as the invention itself, will be more fullyunderstood from the following description of preferred embodiments, whenread together with the accompanying drawings in which:

FIG. 1 provides a graphical representation of angular scattering,wherein backscatter is indicated by a scattering angle of 180°,

FIGS. 2(A)-(D) are schematic representations of the present inventiontranslating apertures and corresponding ks-space representation

FIG. 3(A) is a schematic representation of the present inventiontransducer array defining first transmit and first receive apertures.

FIG. 3(B) is the a schematic representation of the transducer array ofFIG. 3(A) defining second transmit and second receive apertures.

FIGS. 4(A)-7(B) are exemplary, schematic, illustrations of alternativetransmit and receive aperture geometric configurations of the presentinvention.

FIGS. 8(A)-(B) are exemplary, schematic, illustrations of alternativetransmit aperture geometries of the present invention.

FIG. 9 shows a schematic block diagram of the present invention angularscatter imaging system using translating apertures.

FIG. 10, provides an algorithm for a preferred embodiment of the presentinvention.

FIG. 11 provides an algorithm of a preferred embodiment of the presentinvention pertaining to the angular scatter analysis provided at step108 of FIG. 10

FIG. 12 provides a schematic diagram of the formation of c- andd-weighted images wherein two sets of echo data (i₁ and i₂) are acquiredusing the aperture geometries.

FIGS. 13(A)-(C) illustrate simulated B-mode, c-weighted, and d-weightedimages, respectively.

FIGS. 14(A)-(D) illustrate probability density functions for speckletargets and a microcalcification in speckle background imaged bystandard B-mode and present invention d-weighted imaging systems.

FIGS. 15(A)-(D) illustrate K-space representations of a variety ofangular scatter measurement/imaging geometries.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an imaging system and method thereof forimaging angular scatter that coherently processes data from multiplescattering angles. The present invention utilizes translating aperturesto acquire data at two or more scattering angles and then processes thisdata to form images depicting angular scatter information. A preferredembodiment of the present invention would use echoes from two angles toform separate images of the common scattering with angle and thedifferential scattering with angle. Use of the translating aperturesimposes a stability in the speckle pattern with angle that allows directcomparison of echoes acquired at different scattering angles. Thisapproach is intended for implementation in broadband phased arrayimaging systems.

For purposes of illustration, a translating aperture was described byTrahey et al. U.S. Pat. No. 5,673,699 entitled “Method and Apparatus forAberration Correction in the Presence of a Distributed Abberrator” anddiscussed by co-inventors W. F. Walker and G. E. Trahey, in SpeckleCoherence and Implications for Adaptive Imaging, Journal of theAcoustical Society of America, vol. 101, pp. 1847-1858, 1997 (hereinafter “Speckle Coherence” article), the entire disclosures of which arehereby incorporated by reference herein. The Trahey U.S. Pat. No.5,673,699 translating angular aperture was developed as a method toimprove phase aberration correction performance.

Another example of a translating aperture for the correction of phaseaberration is described by G. C. Ng, P. D. Freiburger, W. F. Walker, andG. E. Trahey, in A speckle target adaptive imaging technique in thepresence of distributed aberrations, IEEE Trans. Ultrason. Ferroelec.Freq. Contr., vol. 44, pp. 140-151, 1997, (hereinafter “Speckle TargetAdaptive” article).

Similarly, another type of phase aberration correction method isproposed by D. Rachlin in U.S. Pat. No. 5,268,876, entitled Method ofEstimating Near Field Aberrating Delays, and in Direct estimation ofaberrating delays in pulse-echo imaging systems, JASA, vol. 88, pp.191-198, 1990 (the entire disclosures of which are hereby incorporatedby reference herein) describes transmitting single pulses from each of anumber of individual transmitting elements. Each transmit element ispaired with a receive element such that each transmit/receive pairshares a common midpoint. Receive echoes are then correlated to estimatetime delays which are in turn processed using a matrix formulation toestimate an aberration profile.

Y. Li also proposes a phase aberration correction technique in Li et al.U.S. Pat. No. 5,566,675, entitled Beamformer for Phase AberrationCorrection and as discussed in Phase Aberration Corrections and inAlgorithm Using Near-Field Signal Redundancy Method: Algorithm, inTwentieth International Symposium on Ultrasonic Imaging and TissueCharacterization, vol. Ultrasonic Imaging 17, M. Linzer, Ed. Rosslyn,Va., 1995, pp. 64 (the entire disclosures of which are herebyincorporated by reference herein). Li, like Rachlin, acquires data usingcommon midpoint transmit/receive element pairs and combines delayestimates using a matrix formulation. Like Rachin, Li generates eachtransmit pulse from a single transmit element without focusing orsteering. However, instead of using a single array element, the presentinvention provides a large focused aperture. As such, the use of a largeaperture in the present invention will improve the electronic signal tonoise ratio (SNR) and increases correlation between received signals byrestricting the area of tissue insonified.

Referring to FIGS. 2(A)-(D), as a simplified illustration of one of thefunctions of the present invention, an initial set of echo data isacquired with a transmitter aperture 10 and a receiver aperture 20located in the same physical space, both steered straight ahead, asshown in FIG. 2(A). A second set of echo data is then acquired with thetransmit aperture 10 and receive aperture 20 displaced by equal amountsin opposite directions along the translational axis 5, as shown in FIG.2(C). Note that both arrays remain steered and focused on the originaltarget point 4. It is intended that the transmitter and receiverapertures 10, 20 be implemented with a phased array, such as atransducer array 6 so that all steering, focusing, and aperturetranslation can be performed electronically, i.e. spatially andtemporally. Assuming omnidirectional scattering from the target point 4,the two sets of echoes acquired by the receiver aperture 20 will benearly identical, at least near the focus. FIG. 2(C) also discloses theangle of interrogation as depicted by θ which is defined by the angleformed between the transmit pulse 20 and the receive pulse 30.

Further, for purposes of simplicity an aspect of the present inventioncan be illustrated in “k-space”, which is a frequency domain descriptionof imaging systems and targets as described by the co-inventors W. F.Walker and G. E. Trahey, in The Application of K-Space in MedicalUltrasound, IEEE Transactions on Ultrasonics, Ferroelectrics, andFrequency Control, vol. 45, pp. 541-558, 1998, the entire disclosure ofwhich is hereby incorporated by reference herein. For clarity we willconsider a simplified version of k-space, k_(s)-space. Although thek_(s)-space representation of an imaging system neglects some subtleeffects described in a full k-space representation, it is a good firstorder analysis. The k_(s)-space representation of an imaging system isequal to the convolution of a scaled and reversed version of thetransmit aperture with a scaled and reversed version of the receiveaperture. Thus, for a transmit-receive imaging system 1 with uniformlyweighted apertures of the same size, the k_(s)-space representation is atriangle function, as shown in FIG. 2(B). If both apertures are moved inthe same direction, or if only one is moved, the triangle function willbe shifted to the side. However, if the two apertures are shifted inequal and opposite directions, the triangle function will remain at thesame location, as shown in FIG. 2(D). Thus, by shifting the transmit andreceive apertures in equal and opposite directions it is possible tointerrogate tissue at multiple angles without changing the k_(s)-spacerepresentation. Since the k_(s)-space representation is a FourierTransform of the point spread function (psf), the two imaging geometrieswill also have identical psfs. For omnidirectional scatteringenvironments identical psfs must yield identical speckle patterns. Theability of acquiring nearly identical echoes at multiple angles has beentested and supported by theory, simulation, and experiment as describein the aforementioned “Speckle Coherence” and “Speckle Target Adaptive”articles.

For further explanation, the present invention can also be understood byconsidering the system point spread function (psf). At the focus or inthe far-field of an imaging system the one-way psf is simply a FourierTransform of the aperture. Thus, the psf for a system with a uniformrectangular aperture is a sinc² function. If the aperture is shiftedlaterally then the shift theorem of Fourier Transforms states that thepsf will acquire a linear phase tilt with slope proportional to theshift. The round trip psf is found by taking the product of the transmitand receive psfs. Thus, the psf for an aperture which is centered inspace will be a sinc² function. If both conventional transmit andreceive apertures are shifted in the same direction then the psf willhave a linear phase tilt with twice that expected if only one aperturewere shifted. However, if the apertures are shifted in equal andopposite directions, as in the present invention, then the phase tiltsfrom each one-way psf will cancel and the resultant two-way psf willhave no phase tilt.

Turning now to FIG. 3(A), a transducer array 6 transmits a firsttransmit pulse 21 for each plurality of elements 7 which is focused andfired from a first transmit aperture 11 to the location 4 on the target3 (as depicted by the solid lines). Thereafter, a responsive mode isprovided for receiving the echoes of the pulses transmitted during thetransmission mode. In particular, the transducer array 6 receives afirst echo or pulse 41 at the plurality of elements 7 which are activefor receiving echoes scattered from the location 4 of the target 3 (asdepicted by the broken lines).

Still referring to FIG. 3(A), the elements 51 activated or fired on inthe first transmit 11, as well as having received echoes are illustratedhere in single backward sloping cross-hatch marks. The elements 51 thatare not activated during the pulse and receive sequence are indicated inwhite.

Referring to FIG. 3(B), a subsequent sequence mode of transmission andreception is effected for the target location 4 discussed above. Assuch, the transducer array 6 is operated such that the transmitapertures 11, 12 are translated in equal and opposite directions alongthe translational axis 5 relative to one another in order to obtain theangular scatter results associated with the present invention system 1as described herein. Accordingly, a second transmit pulse 22 for eachelement 7 is then focused and fired (as depicted by the solid lines)from a second transmit aperture 12 that is spatially translated ordisplaced relative to the second receive aperture 32. In particular, theelements 53 fired on in the second transmit aperture 12 are illustratedhere in single backward sloping cross-hatch marks. Whereas, the elements54 activated for receiving the echoes (as depicted by the broken lines)of the second transmit 12 are shown in the second receive aperture 32and are shown as single forward cross-hatch marks.

FIGS. 4(A) through 7(B) are exemplary illustrations of alternativetransmit and receive aperture geometric configurations. There shown foreach respective figure is a first (preceding) iteration oftransmission-reception of an ultrasound signal (referring to respectiveA-Figures) and a second (subsequent) iteration of transmission-receptionof an ultrasound signal (referring to respective B Figures). One skilledin the art would appreciate that the first and second iterations couldbe interchanged as well. It should be noted that the translational axis5 can be oriented at any angle relative to the target location 4.Therefore, in the operating mode of the present invention imaging systemit is contemplated that that the translational axis will be oriented ina plurality of orientations to provide a plurality of distincttranslational axes.

FIGS. 8A and 8B are exemplary illustrations of alternative transmitaperture geometry. A transducer array 6 having a first aperture 11 ofactivated elements and a second aperture 12 of activated elements. Thelegend for the elements is located to the side of the figure. Thetranslation direction is indicated by the arrow. Non-activated elements7 are indicated in white. Of course, these alternatives are merelyexamples of translating transmit apertures and the invention is notlimited thereto.

FIG. 9 shows a schematic block diagram of the present invention angularscatter imaging system using translating apertures which includes atransducer array 6 having a plurality of elements 7 that can be operatedon a plurality of translational axes as discussed above. A transmitter60 operably associated with the transducer array 6 is included forgenerating and transmitting ultrasound pulses 20 at the target 3. Thetransmitter 60 includes a transmit aperture translator 61 fortransmitting pulses to fire from the elements 7 of the transducer array6 thereby defining a transmit aperture 10, whereby the transmit aperturecomprises at least one or more preselected elements 7. Next, a receiver70 is operably associated with the transducer array 6 for receivingechoes of transmitted pulses 40 and outputting echo signals therefrom.The receiver 70 includes a receive aperture translator 72 for receivingpulses 20 transmitted from the subject transmit aperture 10 and receivedat the elements 7 of the transducer array 6 thereby defining a subjectreceive aperture 30, wherein said subject receive aperture comprises atleast one or more preselected said elements.

Still referring to FIG. 9, a controller 82 is provided for controllingthe transmit aperture translator 62 and the receive aperture translator72. As such, as a successive transmission and reception mode isiteratively performed at least twice, the controller 62 effects thetransmit aperture 10 and receive aperture 309 so as to be translatedalong one of a plurality of translation axes 5 in a predetermined equaland opposite direction relative to one another, as discussed in detailherein. Thereafter, the translation may occur after each iteration ofthe transmission-reception mode.

Referring to FIG. 9, a signal processor 84 is operably associated withthe receiver 70 whereby the signal processor 84 is adapted to receivethe echo signals and perform angular scatter analysis on the echosignals after each of the second or subsequent iterations so as toprovide an image signal representative of the target. Also, a beamformer80 is operably associated with the receiver 30 for compensating forgeometric array configurations and target relationships, and for summingechoes from the receive elements. It is contemplated that the beamformer80 can be adapted so as to provide an output to the signal processor 84,or alternatively to receive an output from the signal processor 84.Finally, a detector 88 is provided to perform envelope detection on theprocessed signals and output the detected signals to a display ormonitor 90, printer device, and/or similar display device.

Referring to FIG. 10, an algorithm is provided for a preferredembodiment of the present invention. The algorithm provides steps 100through 106 for implementing the transmission and reception of theultrasound echoes. Thereafter, transmit and receive apertures aretranslated according to step 107. Next, an angular scatter analysis isperformed on the echo signals at step 108 so as to provide an imagesignal representative of the target based on angular scatter accordingto step 200.

Next, FIG. 11 specifically discloses an algorithm of a preferredembodiment of the present invention pertaining to the angular scatteranalysis provided at step 108. In particular, D-weighted and C-weighteddata are derived in steps 110-116 for purpose of displaying therespective D-weighted and C-weighted ultrasound images.

It is contemplated that other preferred embodiments of angular scatteranalysis will be performed by using a method of processing the echoesother than the c-weighted and d-weighted analysis. As discussed above,the c- and d-weighted imaging processing includes, generally, findingthe common (c-) and differential (d-) scattering over two angles ofinterrogation. Radio Frequency (RF) data can then be found by scalingand subtracting operations.

Moreover, an alternative embodiment of the present invention ofprocessing the echo signals may entail “Correlation Imaging”.Correlation imaging essentially calculates the correlation coefficientbetween echo data acquired from different angles.

Further yet, another alternative embodiment of the present invention ofprocessing the echo signals may entail “Ratio Imaging”. Ratio imagingessentially finds the ratio of echoes acquired at different angles. Thiscould be performed on complex demodulated data to simplify computationand reduce the likelihood of errors when the signal drops very low. Itmight be necessary to limit ratios to a certain range for display toeliminate places where the result is unstable.

For purpose of illustration, the present invention can be set forth incontext of FIGS. 2(A) and 2(C) as discussed earlier. In summary, thepresent invention, translation aperture related algorithm, can beexpressed as follows:

1. Acquire echoes using the transducer configuration depicted in FIG.2a.

2. Acquire echoes using the transducer configuration depicted in FIG.2c.

3. Subtract echoes acquired in step 2 from those acquired in step 1 andscale the result to extract the echoes resulting from variation inscattering with angle. (See equation 4 below.)

4. Envelope detect and display the data from step 3 to yield an image ofdifferential angular scattering.

5. Process signals from steps 1 and 3 to yield echoes resulting fromcommon angular scattering. (See equation 5 below.)

6. Envelope detect and display the data from step 5 to yield an image ofthe common angular scattering component.

The image formed in step 4 will highlight the local component ofscattering which differs with angle. This is termed thedifference-weighted or d-weighted image. The image acquired in step 6will highlight the local component of scattering which stays constantwith angle. This is termed the common-weighted or c-weighted image. C-and d-weighted images will offer information that is unavailable instandard B-mode images.

The illustrative algorithm outlined immediately above offers a generalapproach for angular scatter imaging, but lacks the specific scalingfactors need for implementation. In addition, there has not been anyphysical insight into the possible sources of contrast in c- andd-weighted images. These issues are addressed herein by considering ascattering model for the tissue. While the true sources of scattering intissue are undoubtedly very complex, Rayleigh scattering is assumedherein for illustration purposes.

In the instance of Rayleigh scattering, the imaging method describedabove will yield information about the fundamental material propertiesof compressibility and density. This can be shown by considering theacoustic field scattered when an ultrasonic plane wave intersects aspherical Rayleigh scatterer. This is predicted analytically using theequation 1below: $\begin{matrix}{{P\left( {r,\theta} \right)} = {A\quad \frac{^{j\quad k\quad r}}{r}\frac{1}{3}\quad k^{2}{a^{3}\left( {\frac{k_{e} - k}{k} + {\frac{{3\rho_{e}} - 3_{\rho}}{{2\rho_{e}} + \rho}\cos \quad \theta}} \right)}}} & (1)\end{matrix}$

Wherein P(r,θ) is the scattered complex acoustic wave at a range r fromthe target and an angle θ relative to the insonifying direction, A isthe amplitude of the incident plane wave, k is the wave number(k=2π/λ=2πf/c), a is the radius of the scatterer, k_(e) and k are thecompressibilities of the target and the background respectively, andρ_(e) and ρ are the densities of the target and the backgroundrespectively. This expression indicates that the echo generated when aplane wave impinges on a Rayleigh scatterer is equal to the sum of anomnidirectional or monopolar scattered wave resulting from the variationin the scatterer's compressibility from the background and a dipolar orangle dependent scattered wave resulting from the variation in thescatterer's density from the background. Thus an image of angularscatter variations will depict variations in tissue density, while animage of the uniform angular scatter component will depictcompressibility variations. This can be summarized as follows:

common-weighted image=c-weighted image=compressibility-weighted imagedifference-weighted image=d-weighted image=density-weighted image

Rayleigh scattering theory provides a basis for deriving the scalingrequired in step 3 and mathematical operations required in step 5 of thepresent invention imaging methods. This derivation does not restrict c-and d-weighted imaging to Rayleigh scatterers, but does force thesemethods to accurately image compressibility and density variations whenRayleigh scattering dominates. The echo acquired in step 1 can berepresented mathematically as follows:

i₁ =B(γ_(k)−γ₉₂)  (2)

where i₁ is the first received echo, and${B = {A\quad \frac{^{j\quad k\quad r}}{r}\frac{1}{3}\quad \kappa^{2}a^{3}}},{\gamma_{k} = \frac{k_{e} - k}{k}},\quad {{{and}\quad \gamma_{\rho}} = {\frac{{3\rho_{e}} - {3\rho}}{{2\rho_{e}} + \rho}.}}$

This expression was derived by substituting a scattering angle of 180°into equation 1. Note that although the above problem is analyzed with asingle scatterer, the analysis of multiple scatterers could be performedby the application of superposition. The echo acquired in step 2 issimilar, however we now consider a general scattering angle of θ:

i ₁ =B(γ_(k)+γ_(ρ) cos θ)  (3)

where i₂ is the second received echo. From these expressions the densityweighted echo desired in step 3 can be derived: $\begin{matrix}{i_{d} = {\frac{i_{1} - i_{2}}{{- 1} - {\cos \quad \theta}} = {\frac{{B\left( {\gamma_{k} - \gamma_{\rho}} \right)} - {B\left( {\gamma_{k} + {\gamma_{\rho}\cos \quad \theta}} \right)}}{{- 1} - {\cos \quad \theta}} = {B\quad \gamma_{\rho}}}}} & (4)\end{matrix}$

where i_(d) is the density weighted echo. The compressibility weightedecho of step 5 can also be derived:

i _(c) =i ₁ +i _(d) =B(γ_(k)−γ_(ρ))+Bγ _(ρ) =Bγ _(k)  (5)

where i_(c) is the compressibility weighted echo. The c- and d-weightedechoes of equations 4 and 5 can be envelope detected to yield c- andd-weighted ultrasound images.

Referring to FIG. 12, the process of forming c- and d-weighted images isdiagrammed accordingly. On the left side of the figure one set of radiofrequency (rf) data is acquired at backscatter and a second set isacquired at a scattering angle of θ. The second set of data issubtracted from the first and the result is scaled to yield d-weightedrf data. This data is added to the backscatter rf data to yield thec-weighted rf data. C- and d-weighted rf data are envelope detected anddisplayed. The diagram of the formation of c- and d-weighted imagesusing two sets of echo data (i₁ and i₂) that are acquired using theaperture geometries shown on the left of the figure. In both cases theactive apertures are steered and focused on the same target location.The received radio frequency data is processed as shown to yield c- andd-weighted radio frequency data (i_(c) and i_(d)). This data can beenvelope detected and displayed to form images.

EXAMPLE 1 Angular Scatter Imaging of Mircocalcifications and DiffuseLesions

C- and d-weighted images were simulated to explore their ability tovisualize microcalcifications(MCs) and diffuse lesions. Images wereformed by processing simulated rf echoes received at scattering anglesof 180° and 130°, following the algorithm described above. Receivedechoes at each angle of interrogation were predicted by adding a signalgenerated by convolving a compressibility psf with a compressibilitytarget to a signal generated by convolving a density psf with a densitytarget. Image regions corrupted by edge effects were eliminated.Scatterers were placed with sufficient density to ensure the formationof fully developed speckle. The image background was assumed to have aratio of local density variability to local compressibility variability(γ_(ρ)/γ_(κ)) of 0.15. This value was chosen based on published data forcalf's liver as discussed by D. K. Nassiri and C. R. Hill, in The Use ofAngular Acoustical Scattering Measurements to Estimate StructuralParameters of Human and Animal Tissues, Journal of the AcousticalSociety of America. vol. 79, pp. 2048-2054, 1986, the entire disclosureof which is hereby incorporated by reference herein. The simulatedtargets included a diffuse positive contrast density lesion, a MC, and adiffuse negative contrast density lesion. The MC was modelled as asingle point scatterer with (γ_(ρ)/γ_(κ)) of −0.91. This value wasgenerated from published data for calcium hydroxyapatite, the majorcomponent of MCs. Diffuse lesions are arbitrary and were generated toillustrate the potential of c- and d-weighted images to offerinformation unavailable in B-mode images.

The density and compressibility scatterers were generated to bestatistically independent of each other. It seems likely that any realtissue structure which differs in compressibility from the backgroundwill also differ in density, and vice versa. If so, some correlationbetween the density and compressibility scattering functions would beexpected. By assuming no correlation, we consider a scenario where thereis no common information in the c- and d-weighted speckle patterns.

Point spread functions for compressibility and density targets atscattering angles of 180° and 130° were predicted using a new simulationtool called PSF as discussed by M. J. McAllister and W. F. Walker, inPSF: A New Ultrasound Simulation Tool, IEEE Transactions on Ultrasonics,Ferroelectrics, and Frequency Control, in preparation. This PSF toolmodels transmit and receive apertures as collections of point sourcesand receivers. These elements can exhibit either omnidirectional ordipolar sensitivity patterns depending upon whether a hard or softbaffle is assumed. Unlike current tools, such as FIELD, as described byJ. A. Jensen and N. B. Svendsen, in Calculation of pressure fields fromarbitrarily shaped, apodized, and excited ultrasound transducers, IEEETransactions on Ultrasonics, Ferroelectrics, and Frequency Control, vol.39, pp. 262-267, 1992, the entire disclosure of which is herebyincorporated by reference herein, targets can exhibit eitheromnidirectional or dipolar radiation patterns depending upon whethercompressibility or density targets are modelled. Point spread functionsare found by superimposing waveforms from all permutations oftransmitters and receivers assuming spherically spreading, broadbandacoustic waves. Since PSF models the transducers as collections ofpoints, it accounts for variations in the interrogation angle whichoccur across the aperture face.

Point spread functions were modelled for 1.0 cm diameter pistontransducers focused at 4.0 cm. Transmitted pulses had a 50% Gaussianbandwidth centered at 10 MHz. Transducer apertures were modelled ascollections of approximately 128 point sources/receivers separated byroughly half a wavelength. Frequency compensation, a technique describedbelow, was employed to improve psf uniformity with angle ofinterrogation. Each psf was simulated over a window extending 1.0 mmaxially by 2.0 mm laterally, with 20 μm sampling.

FIGS. 13(A)-(C) clearly shows the potential of c- and d-weightedimaging. Referring to FIG. 13(A) the B-mode image, which corresponds toa conventional image, fails to depict either the diffuse lesions or theMC.

Whereas, regarding the present invention, referring to FIG. 13(B) thec-weighted image depicts one lesion (left side of figure) marginally andfails to depict either the other diffuse lesion or the MC.

Finally, regarding the present invention, referring to FIG. 13(C) thed-weighted image depicts the first lesion (left side of figure) as wellas the MC (center of figure) and an additional diffuse lesion (rightside of figure).

As shown by FIG. 13, the relative increase in MC contrast in thed-weighted image of the present invention was enough to make that targetdetectable. Note that all images in FIG. 13 were brightened to increasethe visibility of the negative contrast lesion in the d-weighted image.

EXAMPLE 2 Comparing PDFs for MC Detectablity

As FIG. 13 represents a single realization of an ensemble of imageswhich could be formed of a set of targets. In each realization of thisensemble, the speckle pattern would change, possible obscuring the MC.Accordingly, one skilled in the art would appreciate that there exists aprobability density function (pdf) of MC brightness which can becompared to that of background speckle to estimate detectability. Totaloverlap of the two pdfs would indicate that MC detection was impossible,while no overlap would allow for MC detectability. In this subjectexample, 10,000 images were simulated to determine the improvement in MCdetectability which might be expected in d-weighted images provided bythe present invention system and method.

Accordingly, referring to FIGS. 14(A)-(D) the simulated pdfs are showncomparing B-mode to d-weighted. As indicated by the graph of FIGS. 14(A)and(B) the B-mode pdfs show significant overlap between the tissue pdfsand MC pdfs, respectively. Whereas, the d-weighted pdfs of the presentinvention show almost no overlap, as depicted by the graphs in FIGS.14(C) and (D). This analysis predicts that MCs will be easier to detectin d-weighted images than they are in B-mode images.

Variations of the present invention are contemplated. For example, thepresent invention angular scattering system and method may haveapplications beyond biological materials. One skilled in the art wouldappreciate that the present invention can be applied to various mediumssuch as natural materials (e.g., rocks) or artificial or manmadematerials. One application may be for performing non-destructiveevaluations (NDEs) on various compositions, materials, or mechanicalstructures.

Another variation of the present invention utilizes matrix methods suchas those outlined in Haider et al. U.S. Pat. No. 6,063,033, entitledUltrasound Imaging With Higher-order Nonlinearities, the entiredisclosure of which is hereby incorporated by reference herein. As such,the present invention would display such matrix results to indicateangular scatter information.

Accordingly, the present invention angular scatter imaging system andmethod thereof provides numerous advantages. First, the presentinvention provides an effective ultrasound system by imaging angularscatter by coherently processing data from multiple scattering angleswhile still having stability in the speckle pattern with angle thatallows direct comparison of echoes acquired at different scatteringangles using a translating aperture.

Another advantage of the present invention is the applications of c- andd-weighted imaging in soft tissues. The techniques of the presentinvention are valuable for detecting calcification in soft tissues. Anexample of a clinical application would be in breast imaging. Breastcancer screening is currently performed by x-ray mammography, withultrasonic imaging filling an adjunct role as a method fordistinguishing between fluid filled cysts and solid masses, and morerecently for differentiating between malignant and benign lesions.Ultrasound also plays an important role in directing invasive diagnosticprocedures such as needle and core biopsy. The utility of ultrasound islimited however because one of the main mammographic features ofinterest, MCs, are often invisible ultrasonically. While conventionalsystems can sometimes detect MCs, further improvements in visualizationwould significantly increase the utility of ultrasound in breastimaging. The present invention provides for an improved ultrasonicvisualization of MCs thereby increasing the overall utility ofultrasonic imaging by allowing visualization of this diagnosticallyrelevant feature.

A further advantage of the present invention is that it providesimproved MC visualization which in turn would enable registrationbetween mammograms and ultrasound images. Such image registration wouldallow straightforward comparison of images acquired with these differentmodalities and would simplify the performance of invasive diagnostictechniques such as needle and core biopsy.

In yet another advantage of the present invention, the c- and d-weightedimaging would have clinical applications and thus improve theidentification of calcifications in atherosclerotic plaques. It iswidely believed that the components and structure of atheroscleroticplaques are predictive of future rupture. One component of particularinterest is calcified tissue. Intervascular ultrasound (IVUS) iscurrently the gold standard for identifying plaque calcification,however it may exhibit a low sensitivity to this important feature.Calcified regions are typically identified by their high echogenicityand posterior shadowing. Moreover, the posterior shadowing would bedifficult to detect for small calcifications under conventional methods.The present invention c- and d-weighted imaging would improve IVUS'sensitivity to calcification. This would enable treatment planning whichis custom tailored to specific plaque characteristics.

Further yet, another advantage of the present invention is related tothe fact that the aperture geometries of the c- and d-weighted imagingare easily implemented in an IVUS system by cutting a single elementcylindrically focused in elevation into three sections in elevation. Forexample, the middle section would be used alone to acquire backscatterdata and the outer sections would be used to acquire angular scatterdata.

An additional advantage of the present invention is related to the factthat the c- and d-weighted imaging in IVUS systems can prove to beeasier than in traditional imaging environments because of uniformattenuation (within the blood) and a lack of phase aberrations.

What is claimed is:
 1. A system for imaging a target comprising: atransducer array having a plurality of elements aligned along at leastone of a plurality of translational axes wherein said plurality oftranslational axes are directed horizontally, vertically, and/ordiagonally relative to the target; a transmitter for generating andtransmitting ultrasound pulses at the target operably associated withsaid transducer array; a transmit aperture translator electricallyassociated with said transmitter for transmitting pulses to fire fromsaid elements of said transducer array thereby defining a subjecttransmit aperture, wherein said subject transmit aperture comprises atleast two preselected said elements; a receiver for receiving echoes oftransmitted pulses operably associated with said transducer array andoutputting echo signals therefrom; a receive aperture translatorelectrically associated with said receiver and for receiving pulsestransmitted from said subject transmit aperture and received at saidelements of said transducer array thereby defining a subject receiveaperture, wherein said subject receive aperture comprises at least twopreselected said elements; a controller for controlling said transmitaperture translator and said receive aperture translator wherein thetransmission and reception are iteratively performed at least twice,wherein after each of the iterations of transmission and reception saidsubject transmit aperture and said subject receive aperture aretranslated along one of said plurality translation axes in apredetermined equal and opposite direction relative to one another; anda signal processor operably associated with said receiver, said signalprocessor adapted to receive said echo signals and perform angularscatter analysis on the echo signals after each of the second orsubsequent iterations so as to provide an image signal representative ofthe target.
 2. The system according to claim 1, further comprising: abeamformer operably associated with said receiver for compensating forgeometric array configurations and target relationships, and for summingechoes from said receive elements.
 3. The system according to claim 2,wherein said beamfarmer is applied prior to said signal processing. 4.The system according to claim 2, wherein said beamfarmer is appliedafter said signal processing.
 5. The system according to claim 1,further comprising: a beamformer placed in a signal path before saidsignal processor and operably associated with said receiver forcompensating for geometric array configurations and targetrelationships, and for summing echoes from said receive elements.
 6. Thesystem according to claim 1, further comprising: a beamformer placed ina signal path after said signal processor and operably associated withsaid receiver for compensating for geometric array configurations andtarget relationships, and for summing echoes from said receive elements.7. The system according to claim 1, wherein said transducer arraycomprises said elements arranged in a line, wherein said transducerarray is a linear array device, phased linear array device, or phasedarray device.
 8. The system according to claim 1, wherein saidtransducer array comprises said elements arranged along a curve, whereinsaid transducer array is a curvilinear array device or curvilineartransducer device, whereby said transmit aperture and receive apertureare applied to the curve.
 9. The system according to claim 1, whereinsaid transducer array comprises said elements arranged in parallel rows,wherein said transducer array is a 1.5-D array device.
 10. The systemaccording to claim 1, wherein said transducer array comprises saidelements arranged in a two dimensional plane or surface, wherein saidtransducer array is a 2-D array device.
 11. A method of imaging a targetcomprising the steps of: a) providing a transducer array having aplurality of elements aligned along at least one of a plurality oftranslational axes wherein said plurality of translational axes aredirected horizontally, vertically, and/or diagonally relative to thetarget; b) generating a subject ultrasound pulse at preselected elementsof said plurality of elements to define a subject transmit aperture; c)focusing said subject ultrasound pulse to a predetermined point on thetarget; d) transmitting said subject ultrasound pulse from said subjecttransmit aperture to said target point; e) receiving echoes of saidtransmitted pulses at preselected elements of said plurality of elementsto define a subject receive aperture; f) outputting echo signalsreceived from said receive aperture; g) repeating step “a” through step“f” at least one or more times, wherein after each repetition saidmethod further comprises the additional step of: translating saidsubject transmit aperture and said subject receive aperture along one ofsaid plurality of translation axes in a predetermined equal and oppositedirection relative to one another; and h) processing said echo signalsto perform angular scatter analysis on the echo signals after the firstor subsequent repetitions so as to provide an image signalrepresentative of the target.
 12. The method according to claim 11,further comprising the steps of: focusing and summing echoes from saidreceive elements.
 13. The method according to claim 12, wherein: saidfocusing and summing echoes from said receive elements occur beforesignal processing.
 14. The method according to claim 12, wherein: saidfocusing and summing echoes from said receive elements occur aftersignal processing.
 15. The method according to claim 11, wherein saidprocessing step to perform angular scatter analysis on the echo signalsafter the first or subsequent repetitions so as to provide an imagingsignal comprises the following steps of: a) subtracting said echosignals received during subsequent repetition from said echo signalsreceived during first repetition to provide derived echoes; and b)scaling said derived echoes by a factor of about 1 or greater to provideD-weighted echo data.
 16. The method according to claim 15, furthercomprising the steps of: a) focusing and summing said D-weighted echodata; and b) displaying said focused and summed data to provide aD-weighted ultrasound image.
 17. The method according to claim 15,further comprising the step of: subtracting said derived echoes fromsaid echo signals first received during first repetition to provideC-weighted echo data.
 18. The method according to claim 17, furthercomprising the steps of: a) focusing and summing said C-weighted echodata; and b) displaying said focused and summed C-weighted data toprovide a C-weighted ultrasound image.
 19. The method according to claim11 wherein said signal processing comprises calculating the correlationcoefficient between echoes received at different interrogations angles.20. The method according to claim 11 wherein said signal processingcomprises calculating the ratio of echoes received at differentinterrogations angles.
 21. The method according to claim 11 wherein echodata is acquired at three or more angles and signals are processed toyield information about angular scatter.
 22. The method according toclaim 21 wherein said signal processing comprises fitting a polynomialcurve to the echo data received as a function of angle and theparameters of said polynomial curve are displayed to indicate angularscatter information.
 23. The method according to claim 21 wherein saidsignal processing comprises calculating certain statistics of the echodata received as a function of angle such as the mean and the standarddeviation, and displaying said statistics to indicate angular scatterinformation.
 24. The method according to claim 21 wherein said signalprocessing comprises utilizing matrix methods and displaying said matrixmethod results to indicate angular scatter information.
 25. The methodaccording to claims 11 or 21 wherein said signal processing comprisesprocessing echo information at multiple frequencies to discernvariations in angular scatter behavior as a function of frequency anddisplaying said frequency dependent angular scatter information.
 26. Themethod according to claims 11 or 21 wherein said received echoes areacquired using non-linear imaging methods selected from the groupconsisting of a pulse inversion method and a receiving at differentfrequencies from transmission method.
 27. The method according to claims11 or 21 wherein said angular scatter images are formed at differentfrequencies and the resultant angular scatter images formed at differentfrequencies are averaged using a frequency compounding method.
 28. Themethod according to claims 11 or 21 wherein two or more angular scatterimages are formed with the apertures located at different positions andaveraged to reduce the appearance of speckle using a spatial compoundingmethod.
 29. The method according to claims 11 or 21 wherein saidreceived echoes are scaled, either individually or after summation, tocompensate for the differential effects of frequency dependentattenuation.
 30. The method according to claims 11 or 21 wherein saidreceived echoes are scaled on a frequency dependent basis, eitherindividually or after summations, to compensate for the differentialeffects of frequency dependent attenuation.
 31. The method according toclaims 11 or 21 wherein said received echoes are acquired using multipletranslational axes.
 32. The method according to claims 11 or 21 whereinsaid received echoes are scaled and/or filtered, either individually orafter summation, to compensate for the effects of the limited angularresponse of practical transducer array elements.
 33. The methodaccording to claims 11 or 21 wherein a phase aberration correctionmethod is applied to the imaging system prior to acquisition of echoesfor angular scatter imaging.
 34. The method according to claims 11 or 21wherein spatially varying weightings are applied to transmit and/orreceive apertures using an apodization method.
 35. The method accordingto claims 11 or 21 wherein the size and/or weightings of the transmitand/or receive apertures are adjusted to compensate for an apparentreduction in the aperture as the angle of interrogation is altered. 36.The method according to claims 11 or 21 wherein the frequency of signaltransmitted is altered with interrogation angle to improve specklecoherence at these multiple angles.